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I understand that Wi-Fi transmits on 2.4 GHz or 5 GHz whereas local ham repeaters transmit around 147 MHz or 450 MHz. What differences would be needed in antennas tuning into either of these frequencies, to account for the difference in frequencies, either Wi-Fi or the ham repeaters?

For example, let's say I'm getting or building a Yagi antenna to pick up a distant Wi-Fi signal better, and I also want a Yagi antenna to better listen to (even better if also transmit) a distant ham repeater station. Could I use the same Yagi antenna for both purposes, or if not, how should the two Yagi antennas differ to best work with the different purposes?

In that example, let's say for Wi-Fi I'd plug the antenna into a USB wireless adaptor in a PC, whereas for the ham repeaters I'd plug the antenna into a transceiver, e.g. a simple Baofeng HT.

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In order to use the same antenna design at different frequencies, "all you need to do" is scale all elements of the antenna proportionally to the difference in wavelength. For example, if you take a Yagi antenna designed for around 150 MHz, and scale all of the lengths in its design down by half, you will have a Yagi antenna good for 300 MHz.

Of course, there's no “resize” button on physical objects, so you have to build two separate antennas instead. And there may be mechanical limitations (e.g. the thickness of sufficiently strong wires) that mean a design doesn't physically scale perfectly.

If you use an antenna at a frequency/wavelength that is too far off from what it was designed for, two things will happen:

  • It will have a different impedance at the feed point (the connection to the transmitter). This means that the RF energy will not be efficiently transferred into/out of the antenna, and it may damage a transmitter. (For receivers, this is not usually critical.)

  • The radiation pattern will be not as designed. Generally, a directional antenna will not be as directional — instead of "pointing" in one direction, it will have a spiky pattern with many highs and lows.

It is not impossible to have a directional antenna which works for two different frequency bands. But it must be designed specifically for that pair. And unless physical space is an issue (as with HF antennas which work with long wavelengths and therefore are themselves quite large), it is simpler to use two separate antennas with standard designs, if a suitable multi-band design is not already available.

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